Radar level gauges are commonly used for the continuous determination of filling levels of filling materials, such as liquids or bulk materials, in container, such as a tank or other vessels. In such measurement systems, the filling level is determined based on a delay or time difference between a signal transmitted towards the surface of the filling material, and a received signal reflected from said surface. For the determination of the desired wave delay, there are various radar principles. One of these is the impulse delay method (pulse radar method), another is the frequency modulated continuous wave (FMCW) radar method. In the FMCW radar method, the delay is determined in an indirect manner by transmitting a frequency modulated signal and creating a difference between the transmitted and the received momentary frequency. The pulse radar method, on the other hand, uses the radiation of short microwave pulses, also known as bursts, wherein the direct time duration is determined between the transmission and the reception of the individual pulses.
When using a single antenna for both transmitting and receiving, it is necessary to separate the electrical transmitting signals sent from the microwave generator to the antenna and the receiving signals sent back from the antenna to the microwave receiver. Most radar systems include a transmitter (TX), a receiver (RX) including signal processing, an antenna and a RX/TX-coupler enabling RX and TX to share the antenna. The RX/TX-coupler can take many forms, such as a circulator, fast switch, lossy power divider, etc. Most radar level gauges today use a lossy power divider, which gives a simple design at the expense of 8-12 dB degraded sensitivity.
One approach for realizing a transmitting/receiving antenna is the use of an elliptically polarized wave or, as a special case, a circularly polarized wave, instead of a linearly polarized wave. With an elliptically polarized wave, the electromagnetic field intensity vector spirals along the propagation direction in a helix with an elliptical cross section, whereby the field intensity vector results from superimposing two wave components having different amplitudes. In the special case of a circularly polarized wave, the electromagnetic field intensity vector also spirals along the propagation direction. The spiral, however, has a circular cross section and the two wave components combining to form the resulting wave have equal amplitudes. Depending on the sense of rotation of the spiraling, a distinction can be made between clockwise and anticlockwise (i.e., counter-clockwise) circular polarization, generally referred to as RHCP (right hand circular polarization) and LHCP (left hand circular polarization). RHCP/LHCP are orthogonal, approximately independent, in the same sense as two linear polarizations which are perpendicular (horizontal/vertical, etc), and as is known for those verse in the art, any mixture of linear and circular polarizations are known as elliptical for which pairs of orthogonal polarizations can be chosen.
Use of elliptic/circular polarization for radar level gauging is very useful for two reasons. First, such polarized waves change their rotation during reflection, so that a twice reflected wave has a rotation opposite to that of a once reflected wave. Thus, sensors can distinguish between various reflection components and may suppress undesirable components such as those deflected by the vessel wall. Furthermore, the use of the elliptic/circular polarization may be useful for the determination of the filling level of a bulk material in a vessel when the bulk material has an irregular and fissured surface, e.g. due to turbulence. Depending on the use, an even more important property of the use of two orthogonal polarizations, such as two counter-rotating circular polarizations, is that it allows two antenna functions to be combined in one physical antenna structure while maintaining a certain insulation between the two antenna functions. The dual antenna functions is a way to avoid the lossy power divider of a TX/RX-coupler and thus a way to gain at least 6 dB, but in practice 8-12 dB, in a two way power budget.
Radar level gauges using elliptically/circularly polarized waves are previously known, e.g. by U.S. Pat. No. 5,543,720 and U.S. Pat. No. 6,987,481.
The radar level gauge disclosed in U.S. Pat. No. 5,543,720 is related to a solution where a dielectric disc is inserted within a hollow waveguide in order to generate circularly polarized waves. However, this solution has some practical drawbacks, such as being rather expensive and requiring considerable space. Furthermore, this known solution has relatively high losses and a relatively low bandwidth is provided.
The radar level gauge disclosed in U.S. Pat. No. 6,987,481 comprises two feeding probes arranged in a waveguide, and fed by a directional coupler, having e.g. a 90 degree hybrid coupler for phase separation of the signals. The feeding probes and the directional coupler may be arranged on a printed circuit board, and be arranged in the hollow waveguide. However, this known system is also subject to severe losses, and in particular leakage between the transmitting and receiving probes, whereby the bandwidth becomes relatively low.
There is therefore still a need for a radar level gauge for using elliptically or circularly polarized waves, and where the above-discussed drawbacks in terms of bandwidth, cost and power losses may be eliminated or alleviated.